1 # Towers of Hanoi in sed. 2 # 3 # from: @(#)hanoi.sed 8.1 (Berkeley) 6/6/93 4 # $NetBSD: hanoi.sed,v 1.3 1997/01/09 20:21:35 tls Exp $ 5 # 6 # 7 # Ex: 8 # Run "sed -f hanoi.sed", and enter: 9 # 10 # :abcd: : :<CR><CR> 11 # 12 # note -- TWO carriage returns, a peculiarity of sed), this will output the 13 # sequence of states involved in moving 4 rings, the largest called "a" and 14 # the smallest called "d", from the first to the second of three towers, so 15 # that the rings on any tower at any time are in descending order of size. 16 # You can start with a different arrangement and a different number of rings, 17 # say :ce:b:ax: and it will give the shortest procedure for moving them all 18 # to the middle tower. The rules are: the names of the rings must all be 19 # lower-case letters, they must be input within 3 fields (representing the 20 # towers) and delimited by 4 colons, such that the letters within each field 21 # are in alphabetical order (i.e. rings are in descending order of size). 22 # 23 # For the benefit of anyone who wants to figure out the script, an "internal" 24 # line of the form 25 # b:0abx:1a2b3 :2 :3x2 26 # has the following meaning: the material after the three markers :1, :2, 27 # and :3 represents the three towers; in this case the current set-up is 28 # ":ab : :x :". The numbers after a, b and x in these fields indicate 29 # that the next time it gets a chance, it will move a to tower 2, move b 30 # to tower 3, and move x to tower 2. The string after :0 just keeps track 31 # of the alphabetical order of the names of the rings. The b at the 32 # beginning means that it is now dealing with ring b (either about to move 33 # it, or re-evaluating where it should next be moved to). 34 # 35 # Although this version is "limited" to 26 rings because of the size of the 36 # alphabet, one could write a script using the same idea in which the rings 37 # were represented by arbitrary [strings][within][brackets], and in place of 38 # the built-in line of the script giving the order of the letters of the 39 # alphabet, it would accept from the user a line giving the ordering to be 40 # assumed, e.g. [ucbvax][decvax][hplabs][foo][bar]. 41 # 42 # George Bergman 43 # Math, UC Berkeley 94720 USA 44 45 # cleaning, diagnostics 46 s/ *//g 47 /^$/d 48 /[^a-z:]/{a\ 49 Illegal characters: use only a-z and ":". Try again. 50 d 51 } 52 /^:[a-z]*:[a-z]*:[a-z]*:$/!{a\ 53 Incorrect format: use\ 54 \ : string1 : string2 : string3 :<CR><CR>\ 55 Try again. 56 d 57 } 58 /\([a-z]\).*\1/{a\ 59 Repeated letters not allowed. Try again. 60 d 61 } 62 # initial formatting 63 h 64 s/[a-z]/ /g 65 G 66 s/^:\( *\):\( *\):\( *\):\n:\([a-z]*\):\([a-z]*\):\([a-z]*\):$/:1\4\2\3:2\5\1\3:3\6\1\2:0/ 67 s/[a-z]/&2/g 68 s/^/abcdefghijklmnopqrstuvwxyz/ 69 :a 70 s/^\(.\).*\1.*/&\1/ 71 s/.// 72 /^[^:]/ba 73 s/\([^0]*\)\(:0.*\)/\2\1:/ 74 s/^[^0]*0\(.\)/\1&/ 75 :b 76 # outputting current state without markers 77 h 78 s/.*:1/:/ 79 s/[123]//gp 80 g 81 :c 82 # establishing destinations 83 /^\(.\).*\1:1/td 84 /^\(.\).*:1[^:]*\11/s/^\(.\)\(.*\1\([a-z]\).*\)\3./\3\2\31/ 85 /^\(.\).*:1[^:]*\12/s/^\(.\)\(.*\1\([a-z]\).*\)\3./\3\2\33/ 86 /^\(.\).*:1[^:]*\13/s/^\(.\)\(.*\1\([a-z]\).*\)\3./\3\2\32/ 87 /^\(.\).*:2[^:]*\11/s/^\(.\)\(.*\1\([a-z]\).*\)\3./\3\2\33/ 88 /^\(.\).*:2[^:]*\12/s/^\(.\)\(.*\1\([a-z]\).*\)\3./\3\2\32/ 89 /^\(.\).*:2[^:]*\13/s/^\(.\)\(.*\1\([a-z]\).*\)\3./\3\2\31/ 90 /^\(.\).*:3[^:]*\11/s/^\(.\)\(.*\1\([a-z]\).*\)\3./\3\2\32/ 91 /^\(.\).*:3[^:]*\12/s/^\(.\)\(.*\1\([a-z]\).*\)\3./\3\2\31/ 92 /^\(.\).*:3[^:]*\13/s/^\(.\)\(.*\1\([a-z]\).*\)\3./\3\2\33/ 93 bc 94 # iterate back to find smallest out-of-place ring 95 :d 96 s/^\(.\)\(:0[^:]*\([^:]\)\1.*:\([123]\)[^:]*\1\)\4/\3\2\4/ 97 td 98 # move said ring (right, resp. left) 99 s/^\(.\)\(.*\)\1\([23]\)\(.*:\3[^ ]*\) /\1\2 \4\1\3/ 100 s/^\(.\)\(.*:\([12]\)[^ ]*\) \(.*\)\1\3/\1\2\1\3\4 / 101 tb 102 s/.*/Done! Try another, or end with ^D./p 103 d 104
Indexes created Sat Sep 20 22:09:52 GMT 2025